A New Inverse Charge Constant On-Time (IQCOT) Control for Improvement of Transient Performance in Multiphase Operations
Todays voltage regulators (VRs) must supply high load current to multi-core central processing units (CPUs) with large slew rate requirements, which necessitates multiphase operations with large phase numbers and fast transient response. Currently, ripple-based constant on-time current mode (COTCM) control is widely used in VR controllers for its excellent small-signal property and light load efficiency. From the transient standpoint, one issue with this ripple-based COTCM is that, in the heavy load step-up transient, the increment of inductor current becomes limited by the on-time and minimum off-time ratio in each cycle, which can create large under-shoot at the output. Moreover, in the case of multiphase operation, the limited pulse-overlapping capability of different phases also becomes an issue at the heavy load step-up transient. Some controllers use an external ramp to improve jittering for noise sensitivity at the output. In this case, pulse overlapping in the transient becomes even more difficult. For most of these cases, controllers use nonlinear controls to force pulse overlapping to occur in the load transient. The problem with these threshold-based nonlinear controls is that they must be optimized with changes in the circuit parameters, i.e., Vout. A new COTCM control, based on the inverse-charge control concept (IQCOT) is proposed in this paper to resolve these limitations by allowing natural and linear Ton extension and pulse overlapping in the load step-up transient, without adding any nonlinear control to the system.
The proposed IQCOT structure (inside the red box) with a two-phase VR is presented, as shown in Fig. 1. As illustrated, the difference between Vc and Isum × Ri is converted into current by using a gm amplifier, and this current is used to charge a capacitor. Then this capacitor voltage (Vramp) is compared with a fixed threshold voltage (VTH) to create pulse frequency fsw. When Vramp touches VTH, the off-time ends and a fixed on-time (Ton) is started. This method is shown in the steady-state part of the waveforms in Fig 2. In case of a large load step-up transient, when Vc-IL × Ri becomes very large, fsw pulses can occur even before the end of the previous Ton time. In that way, the proposed IQCOT control can achieve a natural pulse-overlapping feature between phases (shown in Fig.2), and thus is able to improve the load step-up transient performance. Since, in each phase, one Ton pulse can occur even before the previous Ton pulse finishes, they can merge together to create a longer on-time in each phase, using the proposed Ton generator. Therefore, the proposed IQCOT structure enables the control to meet the high current with a faster transient response by achieving smooth, natural and linear pulse overlapping in high phase count multiphase operation, along with natural and linear Ton extension at the load step-up transient. Test results are shown in Fig 3. With these two features together, the proposed control can significantly reduce the amount of output capacitance on the board, thus reducing the cost and footprint.